Conservative Solution Project, an independent group, is behind $85 million in TV ads supporting Rubio. Unlike a Super PAC, Conservative Solutions Project doesn't have to disclose its donors because it exists as a tax-exempt social welfare group under section 501(c)(4) of the tax code.

But watchdog groups like Campaign Legal Center argue that these ads are illegal because they are benefiting an individual presidential candidate instead of advancing the general social welfare of society.

The groups requested that the Justice Department launches an investigation into Conservative Solutions Project. And while Conservative Solution Project officials claim that they are "not about any one specific elected official or candidate," Rubio is the only 2016 presidential candidate featured in any of the organization's TV ads that have aired in the early nominating states of Iowa, New Hampshire and South Carolina, as well as on national cable.